There's no single cause of borderline personality disorder (BPD) and it's likely to be caused by a combination of factors.

The things that are likely to contribute to BPD are explained below.

Genetics

Genes you inherit from your parents may make you more vulnerable to developing BPD.

One study found that if one identical twin had BPD, there was a two-in-three chance that the other identical twin would also have BPD.

However, these results have to be treated with caution, and there's no evidence of a gene for BPD.

Problem with brain chemicals

It's thought that many people with BPD have something wrong with the neurotransmitters in their brain, particularly serotonin.

Neurotransmitters are "messenger chemicals" used by your brain to transmit signals between brain cells. Altered levels of serotonin have been linked to depression, aggression and difficulty controlling destructive urges.

Problem with brain development

Researchers have used magnetic resonance imaging (MRI) scans to study the brains of people with BPD. MRI scans use strong magnetic fields and radio waves to produce a detailed image of the inside of the body.

The scans revealed that in many people with BPD, three parts of the brain were either smaller than expected or had unusual levels of activity. These parts were:

the amygdala – which plays an important role in regulating emotions, especially the more "negative" emotions, such as fear, aggression and anxiety

the hippocampus – which helps regulate behaviour and self-control

the orbitofrontal cortex – which is involved in planning and decision making

Problems with these parts of the brain may well contribute to symptoms of BPD.

The development of these parts of the brain is affected by your early upbringing (see below). These parts of your brain are also responsible for mood regulation, which may account for some of the problems people with BPD have in close relationships.

Environmental factors

A number of environmental factors seem to be common and widespread among people with BPD. These include:

being a victim of emotional, physical or sexual abuse

being exposed to chronic fear or distress as a child

being neglected by one or both parents

growing up with another family member who had a serious mental health condition, such as bipolar disorder or a drink or drug misuse problem

A person's relationship with their parents and family has a strong influence on how they come to see the world and what they believe about other people.

Unresolved fear, anger and distress from childhood can lead to a variety of distorted adult thinking patterns, such as: